In recent years, among electronic apparatuses such as a mobile telephone, various terminals which allow digital television broadcast to be watched are on the market, the components of a radio circuit included in the terminals are required to be miniaturized, and loss caused in the components is required to be reduced. Among the components, a receiving circuit which receives digital television broadcast needs a filter which allows, among received signals, frequencies in a wide band (470 to 770 MHz) for digital television broadcast to pass, and which reduces: a high-frequency signal in a frequency band (830 to 840 MHz) for a mobile telephone, transmitted by a mobile telephone terminal; and an interfering wave from other systems. A high-frequency filter using a piezoelectric resonator such as an SAW (Surface Acoustic Wave Device) or an FBAR (Film Bulk Acoustic Resonator) are known as a filter satisfying requirements that, as described above, the passband is a wide band having a fractional bandwidth (bandwidth/frequency) of about 50%, which is an index indicating a frequency resource, and that an attenuation band in which the attenuation amount is large is placed in the vicinity of the passband. A configuration of a high-frequency filter formed by a combination of a notch filter and a low-pass filter will be described below.
FIG. 11 is a block diagram showing a digital television broadcast receiving section 1000 of a mobile telephone terminal of conventional art, which allows digital television broadcast to be watched. As shown in FIG. 11, the digital television broadcast receiving section 1000 of the mobile telephone terminal includes an antenna 1001, a notch filter 1002, a low-pass filter (LPF) 1003, a low noise amplifier (LNA) 1004, and a receiving IC 1005.
A high-frequency signal received via the antenna 1001 is filtered by the high-frequency filter 1006 formed by the notch filter 1002 and the low-pass filter 1003. First, among received high-frequency signals, the notch filter 1002 largely attenuates frequencies in a mobile telephone transmission frequency band of the mobile telephone terminal, and passes, with low loss, frequencies in a frequency band for digital television broadcast. Next, among high-frequency signals filtered by the notch filter 1002, the low-pass filter 1003 largely attenuates frequencies in an interfering wave frequency band of other systems, which band is largely distant from the frequency band for digital television broadcast, and passes, with low loss, the frequencies in the frequency band for digital television broadcast. The high-frequency signals filtered by the notch filter 1002 and the low-pass filter 1003 are amplified, with low distortion, by the low noise amplifier 1004, and then the amplified high-frequency signals are converted to baseband signals by the receiving IC 1005.
FIG. 12 is an equivalent circuit diagram of the notch filter 1002 of conventional art. As shown in FIG. 12, the notch filter 1002 includes an input matching circuit section 1102a, a notch filter section 1103, and an output matching circuit section 1102b. 
The input matching circuit section 1102a includes a series inductor 1106a one end of which is connected to an input terminal 1101a, and a parallel inductor 1107a one end of which is connected to the other end of the series inductor 1106a. Note that the other end of the parallel inductor 1107a is grounded.
The notch filter section 1103 is a π-type three-stage circuit including a series piezoelectric resonator 1104, a first parallel piezoelectric resonator 1105a, and a second parallel piezoelectric resonator 1105b, one end of the first parallel piezoelectric resonator 1105a and one end of the second parallel piezoelectric resonator 1105b being connected to the respective ends of the series piezoelectric resonator 1104. Note that the other end of the first parallel piezoelectric resonator 1105a and the other end of the second parallel piezoelectric resonator 1105b are grounded. In addition, a connection point between the series piezoelectric resonator 1104 and the first parallel piezoelectric resonator 1105a is connected to a connection point between the series inductor 1106a and the parallel inductor 1107a of the input matching circuit section 1102a. 
The output matching circuit section 1102b includes a series inductor 1106b one end of which is connected to an output terminal 1101b, and a parallel inductor 1107b one end of which is connected to the other end of the series inductor 1106b. Note that the other end of the parallel inductor 1107b is grounded. In addition, a connection point between the series inductor 1106b and the parallel inductor 1107b is connected to a connection point between the series piezoelectric resonator 1104 and the second parallel piezoelectric resonator 1105b of the notch filter section 1103.
Each of the characteristics of the series piezoelectric resonator 1104, the first parallel piezoelectric resonator 1105a, and the second parallel piezoelectric resonator 1105b alone has a resonance point at which the impedance is 0, and an antiresonance point at which the impedance reaches infinity. A frequency at the resonance point is a resonance frequency, and a frequency at the antiresonance point is an antiresonance frequency. The notch filter section 1103 is configured such that the resonance frequency of the first parallel piezoelectric resonator 1105a and the second parallel piezoelectric resonator 1105b are substantially equal to the antiresonance frequency of the series piezoelectric resonator 1104. Thus, the notch filter section 1103 has a characteristic of a notch filter, in which an attenuation band is a frequency band between the antiresonance frequency of the series piezoelectric resonator 1104 and the resonance frequency of the first parallel piezoelectric resonator 1105a and the second parallel piezoelectric resonator 1105b. Moreover, in order to perform impedance matching for the low-pass side of the passband, the notch filter 1002 shown in FIG. 12 is such that the input matching circuit section 1102a and the output matching circuit section 1102b are respectively positioned at the input side and the output side of the notch filter section 1103 so as to be symmetrical about the notch filter section 1103.
FIG. 14 is a diagram showing a pass characteristic of the notch filter 1002 of conventional art. FIG. 14(a) is a diagram showing the pass characteristic in a range from −60 to 0 dB, and FIG. 14(b) is a diagram showing the pass characteristic in a range from −1.2 to 0 dB. A pass characteristic 1401 (characteristic represented by a heavy line in FIG. 14) of the notch filter 1002 is a characteristic in which frequencies in a mobile telephone transmission frequency band B (830 to 840 MHz), which is an attenuation band, are largely attenuated. A pass characteristic 1402 (characteristic represented by a thin line in FIG. 14) of the notch filter 1002 is a characteristic in which pass loss of frequencies in frequency band A (470 to 770 MHz) for digital television broadcast, which is the passband, is reduced.
FIG. 13 is an equivalent circuit diagram of the low-pass filter 1003 of conventional art. In the low-pass filter 1003, a first series inductor 1202a, a second series inductor 1202b, and a third series inductor 1202c are connected in series in this order between an input terminal 1201a and an output terminal 1201b. One end of the first parallel inductor 1203a is connected to a connection point between the input terminal 1201a and the first series inductor 1202a, and the other end of the first parallel inductor 1203a is connected to one end of a first parallel capacitor 1204a. One end of the second parallel inductor 1203b is connected to a connection point between the first series inductor 1202a and the second series inductor 1202b, and the other end of the second parallel inductor 1203b is connected to one end of a second parallel capacitor 1204b. One end of the third parallel inductor 1203c is connected to a connection point between the second series inductor 1202b and the third series inductor 1202c, and the other end of the third parallel inductor 1203c is connected to one end of a third parallel capacitor 1204c. One end of the fourth parallel inductor 1203d is connected to a connection point between the third series inductor 1202c and the output terminal 1201b, and the other end of the fourth parallel inductor 1203d is connected to one end of a fourth parallel capacitor 1204d. Note that the other ends of the first parallel capacitor 1204a, the second parallel capacitor 1204b, the third parallel capacitor 1204c, and the fourth parallel capacitor 1204d are grounded.
In the low-pass filter 1003, the first series inductor 1202a, the second series inductor 1202b, and the third series inductor 1202c are connected in series between the input terminal 1201a and the output terminal 1201b. Thus, the low-pass filter 1003 allows a signal to pass as the frequency of the signal becomes low, as indicated by impedance j2π fL (f: frequency, L: inductance value) of an inductor. On the other hand, the first parallel inductor 1203a and the first parallel capacitor 1204a cause series LC resonance when the frequency of a signal is high, and form a first attenuation pole at a resonance frequency represented by 1/{2π√{square root over ( )}(LC)} (L: inductance value, C: capacitor value). Similarly, the second parallel inductor 1203b and the second parallel capacitor 1204b form a second attenuation pole; the third parallel inductor 1203c and the third parallel capacitor 1204c form a third attenuation pole; and the fourth parallel inductor 1203d and the fourth parallel capacitor 1204d form a fourth attenuation pole. Thus, the low-pass filter 1003 has a low-pass filter characteristic which allows signals having frequencies in a low-frequency band to pass, and which has four attenuation poles in a high-frequency band.
FIG. 15 is a diagram showing a pass characteristic of the low-pass filter 1003 of conventional art. FIG. 15(a) is a diagram showing the pass characteristic in a range from −80 to 0 dB, and FIG. 15(b) is a diagram showing the pass characteristic in a range from −4 to 0 dB. A pass characteristic 1501 (characteristic represented by a heavy line in FIG. 15) of the low-pass filter 1003 is a characteristic in which frequencies in a band of other systems, and harmonics, which frequencies and harmonics are interfering waves, are largely attenuated. A pass characteristic 1502 (characteristic represented by a thin line in FIG. 15) of the low-pass filter 1003 is a characteristic in which pass loss of frequencies in the frequency band A (470 to 770 MHz) for digital television broadcast, which is the passband, is reduced. A filter as described above which is used, in a mobile telephone terminal, for receiving digital television broadcast, is disclosed in Patent Document 1, for example.
Moreover, it is disclosed that a receiving apparatus using a low-pass filter causes a filter control section to switch a filter characteristic of a low-pass filter in accordance with purposes (see Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-202136    Patent Document 2: International Publication No. 2007/111311 Pamphlet